Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y8: Ion Interactions and Transport in Ion-Containing Polymers |
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Sponsoring Units: DPOLY Chair: Ralph Colby, The Pennsylvania State University Room: Portland Ballroom 255 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y8.00001: Nonlinear polarization of ionic liquids: theory, simulations, experiments Invited Speaker: Room temperature ionic liquids (RTILs) composed of large, often asymmetric, organic cations and simple or complex inorganic or organic anions do not freeze at ambient temperatures. Their rediscovery some 15 years ago is widely accepted as a ``green revolution'' in chemistry, offering an unlimited number of ``designer'' solvents for chemical and photochemical reactions, homogeneous catalysis, lubrication, and solvent-free electrolytes for energy generation and storage. As electrolytes they are non-volatile, some can sustain without decomposition up to 6 times higher voltages than aqueous electrolytes, and many are environmentally friendly. The studies of RTILs and their applications have reached a critical stage. So many of them can be synthesized - about a thousand are known already - their mixtures can further provide ``unlimited'' number of combinations! Thus, establishing some general laws that could direct the best choice of a RTIL for a given application became crucial; guidance is expected from theory and modelling. But for a physical theory, RTILs comprise a peculiar and complex class of media, the description of which lies at the frontier line of condensed matter theoretical physics: dense room temperature ionic plasmas with ``super-strong'' Coulomb correlations, which behave like glasses at short time-scale, but like viscous liquids at long-time scale. This talk will introduce RTILs to physicists and overview the current understanding of the nonlinear response of RTILs to electric field. It will focus on the theory, simulations, and experimental characterisation of the structure and nonlinear capacitance of the electrical double layer at a charged electrode. It will also discuss pros and contras of supercapacitor applications of RTILs. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 9:12AM |
Y8.00002: Independent tuning of acidity and ionicity in protic ionic liquids and their polymers. Comparing Li$^{+}$ to H$^{+}$ transport Invited Speaker: Protic ionic liquids (PILs) form an interesting and versatile subclass of the low temperature ionic liquid field, the exponential expansion of which, in recent times, is well known. PILs are formed by transfer of protons from a Br{\o}nsted acid to a Br{\o}nsted base, and their properties depend strongly on the free energy change accompanying the transfer (the proton ``energy gap'').\footnote{ Belieres, J.-P.; Angell, C. A., . \textit{J. Phys. Chem. B }\textbf{2007,} 111, 4926 -4937.} An energy level diagram from which this gap can be predicted for different acid base combinations has been derived from aqueous pKa data,\footnote{ Ibid.} and recently shown to be almost quantitative, by direct electrochemical interrogation of a range of PILs.\footnote{ Bautista-Martinez, J. A.; Tangi, L.; Belieres, J.-P.; Zeller, R.; Angell, C. A., \textit{J. Phys. Chem. B }\textbf{2009,} 113, 12586-12593.} Because of the wide variations in possible proton gaps, the ``ionicity'' of the PIL subclass is highly variable. Furthermore, (a) although a ``pH'' cannot be defined in the absence of H$_{2}$O solvent, the equivalent ``activity'' of the proton can be assessed approximately from the above energy diagram, as the mean of acid and base levels, and can be quantified by such metrics as the N-$^{1}$H chemical shift\footnote{ Shuppert, J. W.; Angell, C. A., C. A. Angell and J. W. Shuppert, J. Phys. Chem., 84, 538 (1980). \textit{J. Phys. Chem. }\textbf{1980,} 84, 538.} for the transferred proton, or the corresponding N-H infrared vibration freqency\footnote{ Stoyanov, E. S.; Kim, K.-C.; Reed, C. A., \textit{J. Amer. Chem. Soc. }\textbf{2006,} 128, 8500}: and (b) the PILs can be obtained in polymeric form by having either the base or the acid pendant from a polymer backbone and then protonating or deprotonating the polymer with an appropriate acid or base moiety. We show how, by tuning the proton gap, we can induce different degrees of decoupling of the proton mobility from the backbone (or the neutralizing moiety) to obtain ``dry'' proton conductors. We contrast the mobility of protons obtained in this way with the mobility of Li$^{+}$ ions in fast-ion conducting polymers and glasses. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:48AM |
Y8.00003: Hierarchical Structures in Ion-Containing Polymers Invited Speaker: Ion-containing polymers are currently used as tough thermoplastics primarily. To extend their applications to the electrolytes in batteries, electroactive polymers for actuation, and permselective membranes in fuel cells, the hierarchical structures of the complex polymers must be characterized and controlled. Over the last decade we have develop scanning transmission electron microscopy, X-ray scattering methods, and image simulations to quantify the nanoscale morphologies in these materials. This seminar will present our recent work, wherein we collaborate with various groups who are synthesizing new ion-containing polymers and correlating the morphologies to transport properties. The first example is a series of Li, Na, and Cs-neutralized sulfonated polyester ionomers with well-defined poly(ethylene glycol) spacer lengths. The state of ionic aggregation depends on the cation type, spacer length, and temperature. The second example is binary mixtures of a poly(styrene-b-methyl methacrylate) diblock copolymer and an ionic liquid, where the ordered morphologies profoundly impact the ionic conductivity. The third example is linear polyethylenes with precisely spaced acid groups and a layered hierarchical structure. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:24AM |
Y8.00004: Components of Dielectric Constants of Ionic Liquids Invited Speaker: In this study \textit{ab initio}-based methods were used to calculate electronic polarizability and dipole moment of ions comprising ionic liquids [1]. The test set consisted of a number of anions and cations routinely used in the ionic liquid field. As expected, in the first approximation electronic polarizability volume turned out to be proportional to the ion volume, also calculated by means of \textit{ab initio} theory. For ionic liquid ions this means that their electronic polarizabilities are at least an order of magnitude larger than those of traditional molecular solvents like water and DMSO. On this basis it may seem surprising that most of ionic liquids actually possess modest dielectric constants, falling the narrow range between 10 and 15. The lower than first expected dielectric constants of ionic liquids has been explored in this work via explicit calculations of the electronic and orientation polarization contributions to the dielectric constant using the Clausius-Mossotti equation and the Onsager theory for polar dielectric materials. We determined that the electronic polarization contribution to the dielectric constant was rather small (between 1.9 and 2.2) and comparable to that of traditional molecular solvents. These findings were explained by the interplay between two quantities, increasing electronic polarizability of ions and decreasing number of ions present in the unit volume; although electronic polarizability is usually relatively large for ionic liquid ions, due to their size there are fewer ions present per unit volume (by a factor of 10 compared to traditional molecular solvents). For ionic liquids consisting of ions with zero ($e.g.$ BF$_{4})$ or negligible ($e.g.$ NTf$_{2})$ dipole moments the calculated orientation polarization does not contribute enough to account for the whole of the measured values of the dielectric constants. We suggest that in ionic liquids an additional type of polarization, ``ionic polarization'', originating from small movements of the centre of the charge on the ions might be present. According to our estimations, this ionic polarization contribution to the dielectric constant could be rather significant (between 8 and 10 for some ionic liquids). In collaboration with Douglas R. MacFarlane, School of Chemistry, Monash University. \\[4pt] [1] E. I. Izgorodina, M. Forsyth and D. R. MacFarlane, Phys. Chem. Chem. Phys., 11, 2452, 2009. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 11:00AM |
Y8.00005: Ion Transport and Structural Properties of Polymeric Electrolytes and Ionic Liquids from Molecular Dynamics Simulations Invited Speaker: Molecular dynamics simulations are well suited for exploring electrolyte structure and ion transport mechanisms on the nanometer length scale and the nanosecond time scales. In this presentation we will describe how MD simulations assist in answering fundamental questions about the lithium transport mechanisms in polymeric electrolytes and ionic liquids. In particular, in the first part of the presentation the extent of ion aggregation, the structure of ion aggregates and the lithium cation diffusion in binary polymeric electrolytes will be compared with that of single-ion conducting polymers. In the second part of the talk, the lithium transport in polymeric electrolytes will be compared with that of three ionic liquids ( [emim][FSI] doped with LiFSI , [pyr13][FSI] doped with LiFSI, [emim][BF4] doped with LiBF4). The relation between ionic liquid self-diffusion, conductivity and thermodynamic properties will be discussed in details. A number of correlations between heat of vaporization Hvap, cation-anion binding energy (E+/-), molar volume (Vm), self-diffusion coefficient (D) and ionic conductivity for 29 ionic liquids have been investigated using MD simulations. A significant correlation between D and Hvap has been found, while best correlation was found for -log((D Vm)) vs. Hvap+0.28E+/-. A combination of enthalpy of vaporization and a fraction of the cation-anion binding energy was suggested as a measure of the effective cohesive energy for ionic liquids. [Preview Abstract] |
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